(1) Field of the Invention
The present invention relates to a method for assaying an exposed iron portion of a coated steel plate or a processed product thereof. More particularly, the present invention relates to an assaying method of selectively detecting an exposed iron portion in a coated steel plate such as tinplate (tin-plated steel plate) and effectively determining and evaluating this exposed portion.
(2) Description of the Prior Art
More than 170 years have passed since tinplate was first used as a can-making material. In view of the corrosive actions of the contents, tinplates having a sufficient and necessary amount of plated tin were used for manufacture of cans till quite recently. However, since the price of tin has been increasing and stable supply of tin is not expected, tinplates having a small amount of plated tin are extensively used at the present.
Highly processed cans such as draw-ironed cans have been developed and they have been used for packing contents having a highly corrosive action.
Under such circumstances, a probability of exposure of iron on the can surface is now very high.
Exposure of iron leads to extreme reduction of the life of a can. Accordingly, it is very important that the degree of exposure of iron should be promptly evaluated and occurrence of accidents owing to abnormal dissolution of iron and hydrogen swelling should be prevented.
As the conventional method for assaying and evaluating the metal-exposed portion of a coated can, there is broadly known a so-called Enamel Rater method. Indeed, this method is valuable for measuring all the metal-exposed portions. However, the object of selectively measuring and evaluating the iron-exposed portion among the metal-exposed portions cannot satisfactorily be attained according to this method.
As the conventional method for measuring and evaluating the exposed iron portion in tinplate, there are known an ISV method (Iron Solution Value method) in which a sample is dipped in a solution containing sulfuric acid and hydrogen peroxide as main ingredients and the amount of iron dissolved out is determined and a TCV method in which a sample is dipped in a solution containing acetic acid and hydrogen peroxide as main ingredients and the amount of iron dissolved out is determined. These conventional methods, however, are defective in that since tin also present as the coating layer is simultaneously dissolved out, iron coated with a thin layer of tin is dissolved out and the amount of this iron is included in the amount of exposed iron dissolved out, with the result that the object of precisely evaluating only the exposed iron uncoated portion cannot be attained satisfactorily.
As the method for evaluating a shelf life or the like of an uncoated can, there is known an ATC method in which the tin-iron alloy layer in tinplate is electrochemically assayed. According to this method, the measurement is carried out after the surface tin plating layer has been removed. Therefore, also this method is apparently unsuitable for attaining the above-mentioned object.
As is seen from the foregoing illustration, in each of the known methods for assaying and evaluating an iron-exposed portion in tinplate, tin present as the coating layer is dissolved out or removed. As far as we know, none of the presently available methods is capable of selectively assaying and evaluating an exposed iron portion alone in tinplate without any influence being given to tin present as the coating layer.